Rhesus disease

Question 1

Define rhesus disease.

Answer 1

a.k.a. haemolytic disease of the fetus and newborn (HDFN)

= a condition which results from transplacental passage of maternal antibodies which cause immune haemolysis of fetal/neonatal red blood cells.

Question 2

What are the two ways in which blood groups are defined? Which is most severely associated with severe haemolytic disease?

Answer 2

ABO group = A, B, O, AB

Rhesus system = C, D, E (most clinically important, but comprises >40 other antigens)

Rhesus system is the one most commonly associated with severe haemolytic disease

Question 3

Other than anti-D, what other rhesus antibody causes HDFN?

Answer 3

Anti-D

Anti-c (small c) - but this is much less common than anti-D

Question 4

Where are genes for the rhesus system encoded?

Answer 4

They are coded on two adjacent genes that sit within chromosome one.

Question 5

Describe the inheritance patterns of rhesus system antigens.

Answer 5

On chromosome 1:

• One gene codes for antigen polypeptides C/c and E/e (where c is the allelic antigen of C and e is the allelic antigen of E)
• Other gene codes for the D polypeptide (rhesus antigen). The d (little d) antigen has not been identified so it is probable that women who are D negative lack the antigen altogether, as opposed to those with c or e alleic antigens.

Antigen expression is usually dominant, whereas those who have a negative phenotype are either homozygous for the recessive allele or have a deletion of that gene

Question 6

What are the three steps of rhesus isoimmunisation?

Answer 6

1. A rhesus-negative mother must conceive a baby who has inherited the rhesus-positive phenotype from the father.
2. Fetal cells must gain access to the maternal circulation in a sufficient volume to provoke a maternal antibody response.
3. Maternal antibodies must cross the placenta and cause immune destruction of RBC in the fetus.

Question 7

Why does rhesus disease not usually affect the first pregnancy?

Answer 7

Even if there is a sensitisation event, the primary response is usually weak and consists primarily of IgM (cannot cross placenta)

Subsequent resensitisations will cause a much greater B-cell response with IgG which can cross the placenta into fetal circulation. If present in suffienct quantity then fetal heamolysis will occur.

Question 8

Name 4 potential sensitising events for rhesus disease.

Answer 8

• Miscarriage
• Termination of pregnancy
• Antepartum haemorrhage
• Invasive prenatal testing (chorion villus sampling, amniocentesis and cordocentesis)
• Delivery

Question 9

How common is rhesus disease? Is it more comon in certain populations?

Answer 9

15% of the UK caucaisan population is rhesus negative but two thirds of RhD -ve mothers would be expected to carry a RhD +ve child as 55% of men are Dd heterozygous.

Lower prevalence of RhD -ve in other ethnic groups

Rhesus disease is uncommon in UK but more common where anti-D prophylaxis is not widespread

Question 10

How is the dosage of anti-D post-sensitising event determined?

Answer 10

The exact dose is determined by:

• the gestation at which sensitization has occurred
• the size of the feto-maternal haemorrhage using Kleihauer test of maternal blood

Question 11

What is the Kleinhauer test?

Answer 11

Kleihauer test of maternal blood = determines the proportion of fetal cells present in the maternal sample.

It relies on the ability of fetal red blood cells to resist denaturation by alcohol or acid and it allows calculation of the size of the feto-maternal transfusion and the amount of extra anti-D Ig required.

Question 12

What time frame within the sensitising event can anti-D be given?

Answer 12

• Ideally within 72 hours
• But can sometimes be given up to 10 days after the event

Question 13

What is the management of sensitising events at different stages of pregnancy?

Answer 13

<12 weeks - Dose: 250 IU. Unlikely that sensitization would occur; anti-D only indicated after ectopic pregnancy, molar pregnancy, therapeutic TOP and in uterine bleeding where this is repeated, heavy or associated with abdominal pain.

12 and 20 weeks - Dose: minimum 250 IU within 72 hours + Kleihauer test done to check if further anti-D is necessary.

>20 weeks - Dose: minimum anti-D Ig dose of 500 IU within 72 hours pending Kleihauer test result. Further anti-D can be given if indicated by the Kleihauer test.

Question 14

In practice, why is ABO incompatibility not usually a problem?

Answer 14

Most anti-A and anti-B antibodies are IgM so cannot cross the placenta

In addition, A and B antigens are not fully developed in the fetus

Question 15

List some indications for prophylactic anti-D in the first trimester.

Answer 15

• ectopic pregnancy,
• molar pregnancy,
• therapeutic TOP
• uterine bleeding where this is repeated, heavy or associated with abdominal pain

Question 16

Why is anti-D prophylaxis offered to all rhesus negative women, even without an obvious sensitising event in pregnancy? When is this offered?

Answer 16

A small number of Rhesus-negative women become sensitised in pregnancy despite no clinically obvious sensitising event

UK guidelines:

• Single dose at 28 weeks OR
• Two doses at 28 and 34 weeks

Question 17

What are the signs of fetal anaemia? At what Hb do these become apparent?

Answer 17

Usually US/clinical fatures are not apparent unless fetal Hb is more than 5g/dL less than the mean for that gestation + not obvious until fetal Hb <6g/dL.

• Polyhydramnios
• Enlarged fetal heart
• Ascites and pericardial effusions
• Hyperdynamic fetal circulation (can be detected by Doppler ultrasound by measuring increased velocities in the middle cerebral artery or aorta).
• Reduced fetal movements.
• Abnormal CTG with reduced variability, eventually a ‘sinusoidal’ trace.

Question 18

What pattern on CTG is characteristic of fetal anaemia?

Answer 18

‘Sinusoidal’ trace

Question 19

What is the prognosis for subsequent pregnancies in rhesus-negative mothers who have been previously sensitised?

Answer 19

Worse outcomes with each pregnancy

Question 20

How can hyperdynamic fetal circulation be detected in fetal anaemia?

Answer 20

Doppler ultrasound of middle cerebral artery or aorta - increased velocities of blood flow

Question 21

What is the management of pregnant women who have been rhesus D sensitised in a prevoius pregnancy?

Answer 21

Sensitisation itself is irreversible

(1) If father is D-rhesus-negative - nothing; no risk that the baby will be rhesus positive.

(2) If father is D-rhesus positive or unknown NB: fetus may still be rhesus-negative if father is heterozygous

1. Close monitoring of antibody levels every 2-4 weeks from booking
2. +/- if antibody levels rise –> examine fetus for anaemia
   
   • ​MCA Dopplers peak velocity measurements correlate well with fetal anaemia
3. +/- if fetal anaemia present –> refer to tertiary fetal medicine centre
   
   • Fetal blood transfusion once anaemia confirmed with blood sample OR
   • Delivery

Question 22

What are the routes of administration of fetal blood transfusion?

Answer 22

1. Into the umbilical vein at point of cord insertion - ideally through placenta and not through amniotic sac
2. Into the intrahepatic vein
3. Into peritoneal cavity - not as effective
4. Into fetal heart

Question 23

What are the characteristics of blood given during fetal tranfusion?

Answer 23

Transfused blood is:

1. RhD negative
2. Crossmatched with a maternal sample.
3. Densely packed (haemoglobin usually around 30 g/l) so that small volumes are used.
4. WCC depleted and irradiated.
5. Screened for infection including CMV

Question 24

What is a useful side effect of fetal transfusion in HDFN?

Answer 24

Tansfusion –> suppress fetal erythropoiesis –> reduced oncentration of antigen-positive cells –> less cells available for haemolysis

Question 25

What precautions should be taken at the delivery of an anaemic baby?

Answer 25

1. Neonatologist present
2. Blood ready for exchange transfusion

Cord blood should be taken at delivery for FBC, blood group and indirect Coombs test

Question 26

When can ABO sensitisation occur?

Answer 26

Anti-A and anti-B antibodies are present in the maternal circulation naturally

Usually secondary to sensitisation against A or B substances in food or bacteria

Question 27

When can ABO incompatibility become problematic in pregnancy?

Answer 27

If mother is O and fetus is A or B

This means that anti-A or anti-B antibodies can pass to fetal circulation causing fetal haemolysis and anaemia

This may cause mild haemolytic disease of the newborn OR unexplained jaundice in a healthy newborn

Question 28

How can fetal blood group be determined antenatally non-invasively? What is an advantage of this?

Answer 28

Cell-free fetal DNA (cffDNA) testing in maternal blood - acuracies are close to 100%. cffDNA samples are measured between 16-20 weeks.

This could be used to prevent unnecessary prophylaxis - which is unnecessary in 40% of D negative women in the UK

Question 29

On routine antenatal investigation, a 30-year-old woman is found to be rhesus negative. Which piece of advice regarding the management of her pregnancy is correct? Choose the single best answer.

• A Her fetus will also be rhesus negative.
• B If she experiences vaginal bleeding later in pregnancy, a Keilhauer test should be performed.
• C She should have a routine dose of anti-D at 23 weeks’ gestation.
• D Once she has had two doses of anti-D, further administration will not be required.
• E If this pregnancy is not affected by rhesus disease, there should be no problem in subsequent pregnancies

Answer 29

B

Approximately 15% of the Caucasian population are rhesus negative. The rhesus status of the fetus depends on the father’s blood type. Exogenous anti-D immunoglobulin is administered in an attempt to prevent the manufacture of the endogenous antibody by the mother, as this would sensitize the immune system and put subsequent fetuses at risk, Routine doses of anti-D are normally given between 28 and 34 weeks’ gestation, but should be considered after every sensitizing event. The Keilhauer test determines the portion of fetal cells within the maternal circulation and hence helps determine anti-D dosage.

Question 30

What units are used to measure antibody levels in a previously rhesus-D sensitised rhesus-negative woman? Above which anti-D levels is HDFN likely?

Answer 30

1. IU/ml OR
2. Titre -this refers to the number of times that a sample has been diluted before the amount of antibody becomes undetectable titre of 2, 4, 8, 16, 32, 64, 128, etc.